The genome of most vertebrates and some invertebrates has been transformed over millions of years by the integration of infectious retroviruses. Inheritable integrations into the genome of germ cells may have led to rapid evolutionary changes as the host accommodated and used advantageous retroviral proteins. Approximately 8% of the human genome is of apparent retroviral origin. A small number of endogenous retrovirus (ERV) integration sites retained isolated transcriptionally active open reading frames so that a variety of gag (matrix and capsid), pol (polymerase and other enzymes), and env (envelope) proteins may be expressed. The developing placenta is one of few organs that express ERV proteins under physiologic conditions. ERV3 is a single copy ERV genome with an open reading frame in the env region that is expressed during normal human villous cytotrophoblast differentiation. We determined that ERV3 env expression, through a cAMP/protein kinase A dependent pathway, is essential for expression of -hCG in the syncytiotrophoblast. Results of our preliminary studies indicate that the active site is located in the N-terminal portion of ERV3 Env (referred to the p25 region), a unique finding related to retroviral Env proteins in general. Transfection of siRNA targeted to ERV3 env completely blocks induction of -hCG. Our published and preliminary data support a hypothesis that ERV-3 env expression is essential for normal trophoblast differentiation and implantation through regulation of -hCG expression. The goal of this R21 is to obtain certain critical data must be obtained in support of a future RO1. AIM 1. Mechanism of action of ERV3 Env on -hCG expression. We will investigate the pathway through which ERV3 env expression affects expression of -hCG. We will determine whether this pathway is activated through intracellular complex formation between ERV3 Env and co-factors or accessory molecules. We will also investigate an alternative secreted ERV3 Env-receptor mediated process. AIM 2: Effects of ERV3 env expression of the -hCG promoter. Activation of the -hCG promoter may occur secondary to cAMP-dependent transcription factors or directly if ERV3 Env is a transcription factor. Studies will evaluate both potential processes.